The transforming growth factor .beta. (TGF-.beta.) family of proteins includes the transforming growth factors-.beta. (TGF-.beta.), activins, bone morphogenic proteins (BMP), nerve growth factors (NGFs), brain-derived neurotrophic factor (BDNF), and growth/differentiation factors (GDFs). These family members are involved in the regulation of a wide range of biological processes including cell proliferation, differentiation, and other functions.
Growth/differentiation factor 8 (GDF-8), also referred to as myostatin, is a TGF-.beta. family member expressed for the most part in the cells of developing and adult skeletal muscle tissue. Myostatin appears to play an essential role in negatively controlling skeletal muscle growth (McPherron et al., Nature (London), 387:83-90, (1997); Zimmers et al., Science, 296:1486-1488, (2002)). Antagonizing myostatin has been shown to increase lean muscle mass in animals.
Another member of the TGF-.beta. family of proteins is a related growth/differentiation factor, growth/differentiation factor 11 (GDF-11). GDF-11 has approximately 90 sequence identity to the amino acid sequence of myostatin. GDF-11 has a role in the axial patterning in developing animals (Oh et al., Genes Dev., 11:1812-26, (1997)), and also appears to play a role in skeletal muscle development and growth.
Activins A, B and AB are the homodimers and heterodimer respectively of two polypeptide chains, .beta.A and .beta.B (Vale et al., Nature, 321:776-779, (1986); Ling et al., Nature, 321:779-782, (1986)). Activins were originally discovered as gonadal peptides involved in the regulation of follicle stimulating hormone synthesis, and are now believed to be involved in the regulation of a number of biological activities. Activin A is a predominant form of activin.
Activin, myostatin, GDF-11 and other members of the TGF-.beta. superfamily bind and signal through a combination of activin type IIA (ActRIIA) and activin type IIB (ActRIIB) receptors, both of which are transmembrane serine/threonine kinases (Harrison et al., J. Biol. Chem., 279:28036-28044, (2004)). Cross-linking studies have determined that myostatin is capable of binding the activin type II receptors ActRIIA and ActRIIB in vitro (Lee et al., PNAS USA, 98:9306-11, (2001)). There is also evidence that GDF-11 binds to both ActRIIA and ActRIIB (Oh et al., Genes Dev., 16:2749-54, (2002)).
ActRIIB polypeptides can be prepared as a soluble variant of ActRIIB-Fc. Soluble ActRIIB-Fc potently stimulates muscle growth by sequestering multiple ligands such as myostatin, activin and GDF11 (Lee S J, et al., Proc Natl Acad Sci USA., 102(50):18117-22, (2005 Dec. 13) (Epub. 2005 Dec. 5)). These ligands, including myostatin, bind to two high affinity receptors, ActRIIB and ActRIIA. These two receptors are encoded by two different genes, which encode two distinct transmembrane receptor proteins with about 65% sequence homology at the amino acid level. Ligand binding at the cell membrane to either of these two receptors has been shown to cause the phosphorylation of Smads 2/3 and, as a result, to activate downstream transcriptional changes in the cell, (Lee S J, et al., Proc Natl Acad Sci USA., 102(50):18117-22, (2005 Dec. 13) (Epub. 2005 Dec. 5)). Skeletal muscle cells express both of these receptors. Interfering with the activin receptors, e.g. by using an antagonistic dual receptor antibody can result in physiological effects by blocking the activin signaling pathway.
The present invention provides a biologically active therapeutic that blocks at least activin activity and is thereby capable of stimulating skeletal muscle growth.